1. Field of the Invention
The present invention relates to an electrode for a fuel cell and a membrane-electrode assembly and a fuel cell system including the same. More particularly, the present invention relates to an electrode which is capable of maintaining moisture hygroscopicity of a polymer electrolyte membrane at a predetermined level, releasing water produced at a cathode, thereby, preventing clogging of pores of the membrane by water, and improving a current density at high rate.
2. Description of Related Art
A fuel cell is a power generation system for producing electrical energy through an electrochemical redox reaction of an oxidant and a fuel, such as hydrogen, or a hydrocarbon-based material, such as methanol, ethanol, natural gas, or the like.
The fuel cell can be classified as a phosphoric acid type, a molten carbonate type, a solid oxide type, a polymer electrolyte type, or an alkaline type of fuel cell depending upon the kind of electrolyte used. Although each fuel cell basically operates in accordance with the same principles, the kind of fuel, the operating temperature, the catalyst, and the electrolyte may be selected depending upon the type of cell.
Recently, polymer electrolyte membrane fuel cells (PEMFCs) have been developed. They have power characteristics that are superior to conventional fuel cells, as well as lower operating temperatures, and faster start and response characteristics. Because of this, PEMFCs have a wide range of applications, such as for transportable power sources for automobiles, distributed power sources for residences and public buildings, and small power sources for electronic devices.
Such a fuel cell system includes a stack which substantially generates electricity, the stack including at least one electricity generating element.
The stack for generating the electricity has a structure in which several unit cells, each having a membrane-electrode assembly (MEA) and separators (also referred to as “bipolar plates”), are stacked adjacent to one another in series. The MEA is composed of an anode (referred to also as a “fuel electrode” or an “oxidation electrode”) and a cathode (referred to also as an “air electrode” or a “reduction electrode”) that are separated by a polymer electrolyte membrane. The anode and cathode are composed of a catalyst layer contacting a polymer electrolyte membrane and a gas diffusion layer (GDL) contacting the catalyst layer. Each separator includes a gas flow path for supplying a fuel to the anode and an oxidant to the cathode. Separators positioned at the outermost ends of the stack are end plates.
The separators function both as channels for supplying the fuel and the oxidant required for a reaction to the anode and the cathode, as well as conductors for serially connecting the cathode and the anode in each MEA and connecting the cathode of one MEA to the anode of a neighboring MEA.
Hydrogen or a fuel is supplied to the anode and an oxidant is supplied to the cathode through the separators. The electrochemical oxidation reaction of the fuel occurs at the anode and the electrochemical reduction reaction of the oxygen occurs at the cathode, and as a result of the transfer of the electrons generated by the oxidation/reduction reactions, electrical energy, heat, and moisture are produced. The reaction is as follows.At the anode: H2→2H++2e−or CH3OH+H2O→6H++CO2+6e−At the cathode: 2H++½O2+2e−→H2O
In this reaction scheme, water is produced in the cathode reaction. More water is relatively produced at the oxidant inlet of a separator because there is a higher reaction rate in that region. The produced water should immediately be removed through an outlet. If not, pressure of the oxidant supplied through the separator increases, as does the hygroscopicity of the polymer electrolyte membrane of the membrane-electrode assembly.
Generally, as hygroscopicity of a polymer electrolyte membrane increases, proton conductivity increases. Thus, a polymer electrolyte membrane has to include moisture at a predetermined level. However, excessive moisture may induce clogging of the gas flow of the gas diffusion layer or the separator, and gas diffusion is thereby reduced resulting in deterioration of cell performance. Therefore, in order to manufacture a fuel cell with high performance, a polymer electrolyte membrane should remove an excess amount of moisture safely and rapidly while maintaining a suitable hygroscopic state.